Methods for coating surfaces with magnetic composites exhibiting distinct flux properties

ABSTRACT

A method for coating a surface with a magnetic composite material exhibiting distinct flux properties due to gradient interfaces within the composite. Surfaces coated with such a composite can be used to improve fuel cells and to effect improved transport and separation of different species of materials. A wide variety of devices can incorporate such composite-coated surfaces, including separators, fuel cells, electrochemical cells, and electrodes for channeling flux of, or for effecting electrolysis of, magnetic species.

This application is a division of U.S. application Ser. No. 08/626,082,filed Apr. 1, 1996, now U.S. Pat. No. 5,786,040, which is a division ofU.S. application Ser. No. 08/294,797, filed Aug. 25, 1994, nowabandoned.

Part of the work performed during the development of this inventionutilized U.S. government funds under grants No. CHE92-96013 and No.CHE93-20611 from the National Science Foundation, Chemistry Division,Analytical and Surface Science. The government may have certain rightsin this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method for forming and exploitinggradients at the interfaces between components of a composite materialas well as to a method of coating a surface with a magnetic compositematerial, and devices which incorporate the material. In particular, theinvention relates to a method for forming and exploiting magneticgradients at the interfaces between components of a composite magneticmaterial and the magnetic composite material itself as well as deviceswhich incorporate the composite material including electrochemicalsystems resulting in enhanced and modified flux and performance in thosesystems.

2. Baxckground of the Related Art

Bulk properties of molecules in magnetic fields are fairly wellunderstood. In the detailed description of preferred embodiments, itwill be shown that interfacial gradients in properly prepared compositematerials can be exploited to enhance flux in many types ofelectrochemical systems such as fuel cells, batteries, membrane sensors,filters and flux switches. First, however, the following discussionprovides a brief overview of the current understanding of magneticproperties in composites. In particular, the discussion below summarizesthe thermodynamic, kinetic and mass transport properties of bulkmagnetic materials.

Rudimentary Magnetic Concepts

Paramagnetic molecules have unpaired electrons and are attracted into amagnetic field; diamagnetic species, with all electrons paired, areslightly repelled by the field. Radicals and oxygen are paramagnetic;most organic molecules are diamagnetic; and most metal ions andtransition metal complexes are either para- or diamagnetic. How stronglya molecule or species in a solution or fluid responds to a magneticfield is parameterized by the molar magnetic susceptibility,X_(m)(cm³/mole). For diamagnetic species, X_(m) is between (−1 to−500)×10⁻⁶ cm³/mole, and temperature independent. For paramagneticspecies, X_(m) ranges from 0 to +0.01 cm³/mole, and, once corrected forits usually small diamagnetic component, varies inversely withtemperature (Curie's Law). While ions are monopoles and will either movewith or against an electric field, depending on the sign of the ion,paramagnetic species are dipoles and will always be drawn into (alignedin) a magnetic field, independent of the direction of the magneticvector. The dipole will experience a net magnetic force if a fieldgradient exists. Because electrochemistry tends to involve singleelectron transfer events, the majority of electrochemical reactionsshould result in a net change in the magnetic susceptibility of speciesnear the electrode.

Magnetic field effects on chemical systems can be broken down into threetypes: thermodynamic, kinetic and mass transport. Macroscopic,thermodynamic effects are negligible, although local magnetic fieldeffects may not be. Kinetically, both reaction rates and productdistributions can be altered. Transport effects can lead to fluxenhancements of several-fold. Quantum mechanical effects are alsopossible, especially on very short length scales, below 10 nm. Thefollowing summarizes what has been done with homogeneous fields appliedto solutions and cells with external laboratory magnets.

Thetmodynamics

A magnetic field applied homogeneously by placing a solution between thepoles of a laboratory magnet will have a negligible nonexponentialeffect on the free energy of reaction. ΔG_(m)=−0.5Δχ_(m)B² J/mole, whereΔG_(m) is the change of the free energy of reaction due to the magneticfield, Δχ_(m) is the difference in magnetic susceptibility of theproducts and reactants, and B is the magnetic induction in gauss. Forthe conversion of a diamagnetic species into a paramagnetic species,Δχ_(m)≦0.01 cm³/mole. In a 1 T (1 Tesla=10 kGauss) applied field,|ΔG_(m)|≦0.05 J/mole. Even in the strongest laboratory fields of 10T,the effect is negligible compared to typical free energies of reaction(˜kJ/mole). These are macroscopic arguments for systems where the magnetis placed external to the cell and a uniform field is applied to thesolution. Microscopically, it may be possible to argue that local fieldsin composites are substantial, and molecules in composites within ashort distance of the source of the magnetic field experience stronglocal fields. For example, for a magnetic wire or cylinder, the magneticfield falls off over a distance, x, as x⁻³. The field experienced by amolecule 1 nm from the magnet is roughly 10²¹ times larger than thefield experienced at 1 cm. This argument is crude, but qualitativelyillustrates the potential advantage of a microstructural magneticcomposite. (As an example, in composites comprising magnetic materialand “Nafion”(DuPont), a larger fraction of the redox species areprobably transported through the 1.5 nm zone at the interface betweenthe Nafion and the magnetic particles.) These redox species musttherefore experience large magnetic fields in close proximity to theinterface.

Kinetics

Reaction rates, k, are parameterized by a pre-exponential factor, A, anda free energy of activation, ΔG¹; k=A exp[−ΔG¹/RT]. An externallyapplied, homogeneous magnetic field will have little effect on ΔG¹, butcan alter A. Nonadiabatic systems are susceptible to field effects.Magnetic fields alter the rate of free radical singlet-tripletinterconversions by lifting the degeneracy of triplet states (affectingΔG¹); rates can be altered by a factor of three in simple solvents.Because magnetic coupling occurs through both electronic nuclearhyperfine interactions and spin-orbit interactions, rates can benonmonotonic functions of the applied field strength. Photochemical andelectrochemical luminescent rates can be altered by applied fields. Forsinglet-triplet interconversions, magnetic fields alter productdistributions when they cause the rate of interconversion to becomparable to the rate free radicals escape solvent cages. These effectsare largest in highly viscous media, such as polymer films and micellarenvironments. Larger effects should be observed as the dimensionality ofthe system decreases. For coordination complexes, photochemical andhomogeneous electron transfer rates are altered by magnetic fields.Spin-orbit coupling is higher in transition metal complexes than organicradicals because of higher nuclear charge and partially unquenchedorbital angular momentum of the d-shell electrons. The rate ofhomogeneous electron transfer between Co(NH₃)₆ ³⁺ and Ru(NH₃)₆ ²⁺ isbelow that expected for diffusion controlled reactions; in a 7 Tmagnetic field, the rate is suppressed two to three-fold. It has beenargued that Δχ_(m) (and ΔG_(m)) is set by the magnetic susceptibility ofthe products, reactants, and activated complex, and a highlyparamagnetic activated complex accounts for the field effect. Forreversible electron transfer at electrodes in magnetic fields, nosignificant effect is expected. For quasireversible electron transferwith paramagnetic and diamagnetic species, electron transfer rates andtransfer coefficients (α) are unchanged by magnetic fields appliedparallel to electrodes. Magnetic fields applied perpendicular toelectrodes in flow cells generate potential differences, which justsuperimpose on the applied electrode potentials. Potentials of 0.25Vhave been reported. Reversing the applied magnetic field reverses thesign of the potential difference. This effect does not change standardrate constants, only the applied potential.

Mass Transport

Magnetically driven mass transport effects have been studied inelectrochemical cells placed between the poles of large magnets. Effectsvary depending on the orientation of the electrode, the relativeorientation of the magnetic field and the electrode, forced or naturalconvection, and the relative concentrations of the redox species andelectrolyte. Three cases are illustrated in FIGS. 1, 2 and 3.

For a charged species moving by natural or forced convection parallel toan electrode and perpendicular to a magnetic field which is alsoparallel to the electrode, a Lorentz force is generated which moves thecharged particle toward the electrode (FIG. 1). This magnetohydrodynamiceffect is characterized by

F=q(E+v×B),  (1)

where F, E, v, and B are vectors representing the Lorentz force on thecharged species, the electric field, the velocity of the moving species,and the magnetic field, respectively; and q is the charge on thespecies. For flow cells and vertical electrodes, flux enhancements of afew-fold and reductions in the overpotential of a few tenths volts havebeen found in the presence of the magnetic field, Also, embedded inEquation 1 is the Hall effect; when a charged species moves through aperpendicular magnetic field, a potential is generated. This potentialsuperimposes on the applied potential and causes migration in lowelectrolyte concentrations.

When the electrode and magnetic field are parallel to the earth, thermalmotion leads to vortical motion at the electrode surface unless thefield (B) and the current density (j) are spatially invariant andmutually perpendicular (see FIG. 2). This is parameterized as:

 F _(v) =c ⁻¹ [j×B].  (2)

In Equation (2) F_(v) is the vector of magnetic force per volume and cis the speed of light. In general, these forces are smaller than Lorentzforces; flux enhancements of a few-fold and potential shifts of 10 to 20mV are observed. Flux enhancements of paramagnetic and diamagneticspecies are similar, but paramagnetic electrolytes enhance the flux ofdiamagnetic Zn²⁺ two-fold. Vortices suppress thermal motion and eddydiffusion.

The final configuration, shown in FIG. 3, is for the magnetic fieldperpendicular to the electrode surface, and, therefore, parallel to theelectric field. Various, and sometimes inconsistent, results arereported for several configurations: for vertical electrodes inquiescent solution, flux enhancements of ≦1000%; for electrodes parallelto the earth with forced convection, flux retardations of 10%; and forelectrodes parallel to the earth and no forced convection, bothenhancements and no enhancements are reported.

The above summarizes the thermodynamic, kinetic, and mass transporteffects for systems where the magnetic field is applied uniformly acrossa cell with an external magnet. None of these macroscopic effectspredict or address properties dependent on the magnetic susceptibilityof the redox species. Quantum mechanical effects may also be important,especially on short length scales.

Fuel Cells

Since the incomplete reduction of oxygen limits the efficiency of H₂/O₂solid polymer electrolyte fuel cells, the cathode must be pressurizedabout five-fold over the anode.

Proton exchange membrane (PEM) fuel cell design is one which employshydrogen as an anode feed and oxygen in air as a cathode feed. Thesefuels are decomposed electrically (to yield water) at electrodestypically modified with a noble metal catalyst. The hydrogen and oxygenare separated from each other by a proton exchange membrane (such asNafion) to prevent thermal decomposition of the fuels at the noble metalcatalysts. $\begin{matrix}{Cathode} \\{Anode} \\{{Net}\quad {Reaction}}\end{matrix}\begin{matrix}{{O_{2} + {4\quad H^{+}} + {4\quad e}} = {2\quad H_{2}O}} \\{{{2\quad H^{+}} + {2\quad e}} = H_{2}} \\{{O_{2} + {2\quad H_{2}}} = {2\quad H_{2}O}}\end{matrix}\begin{matrix}{E_{cathode}^{o} =} & {1.23V} \\{E_{cathode}^{o} =} & {0.00V} \\{E_{cell}^{o}\quad =} & {1.23V}\end{matrix}$

However, fuel cell is typically run under non-equilibrium condition,and, as such, is subject to kinetic limitations. These limitations areusually associated with the reaction at the cathode.

O₂+4H⁺+4e=2H₂ O E^(o) _(cathode)=1.23V

As the reaction at the cathode becomes increasingly kinetically limited,the cell voltage drops, and a second reaction path, the two electron/twoproton reduction of oxygen to peroxide, becomes increasingly favored.This consumes oxygen in two electron steps with lower thermodynamicpotential.

O₂+2H⁺+2e=H₂O₂ E^(o) _(H) ₂ _(Odi 2)=0.68V

The standard free energy of this reaction is 30% of the free energyavailable from the four electron reduction of oxygen to water. Thedecrease in current associated with the decreased number of electronstransferred and the decreased cell potential couple to yieldsubstantially lower fuel cell power output.

One approach to enhance the efficiency of the cathodic reaction is toincrease the concentration (pressure) of the feeds to thecathode—protons and oxygen—so as to enhance the flux (i.e., the reactionrate at the cathode in moles/cm²s¹) at the cathode. The proton flux isreadily maintained at a sufficiently high value by the proton exchangemembrane (usually Nafion) so as to meet the demand set by the cathodereaction. Normally, the method of enhancing the flux and biasing thereaction to favor the formation of water is to pressurize the air feedto the cathode. Pressures of 5-10 atmospheres are typical.

The need to pressurize air to the cathode in PEM fuel cells has been amajor obstacle in the development of a cost effective fuel cell as areplacement for the internal combustion engine e.g. in a vehicle. Inparticular, pressurization of the cathode requires compressors. Intransportation applications, power from the fuel cell is needed to runthe compressor. This results in approximately 15% reduction in the poweroutput of the total fuel cell system.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a process forcoating a surface of a device with a magnetic composite materialresponsive to an external magnetic field.

Another object of the invention is to provide a process for coating asurface of a device with a magnetic composite material having aplurality of boundary regions with magnetic gradients having paths tothe coated surface when an external magnetic field is applied to thecoated surface.

Another object of the invention is to provide a coating on an electrodeto enhance the flux of magnetic species to the electrode.

Another object of the invention is to provide a separator to separatemagnetic species from each other dependent upon magnetic susceptibility.

Another object of the invention is to provide a method for making acoating for an electrode to improve the flux of magnetic species to theelectrode.

Another object of the invention is to provide an improved fuel cell.

Another object of the invention is to provide an improved cathode in afuel cell.

Another object of the invention is to provide an improved battery.

Another object of the invention is to provide an improved membranesensor.

Another object of the invention is to provide an improved flux switch.

Another object of the invention is to provide an improved fuel cellcathode with passive oxygen pressurization.

Another object of the invention is to provide an improved separator forseparating paramagnetic species from diamagnetic species.

Another object of the invention is to provide an improved electrolyticcell.

Another object of the invention is to provide an improved electrolyticcell for an electrolyzable gas.

Another object of the invention is to provide an improved graded densitycomposite for controlling chemical species transport.

Another object of the invention is to provide an improved dual sensor.

One advantage of the invention is that it can enhance the flux ofparamagnetic species to an electrode.

Another advantage of the invention is that it can enhance the flux ofoxygen to the cathode in a fuel cell, equivalent to passivepressurization.

Another advantage of the invention is that it can separate paramagnetic,diamagnetic, and nonmagnetic chemical species from a mixture.

Another advantage of the invention is that it can separate chemicalspecies according to chemical, viscosity, and magnetic properties.

Another advantage of the invention is that it can take advantage ofmagnetic field gradients in magnetic composites.

Another advantage of the invention is that it can be designed to workwith internal or external magnetic fields, or both.

One feature of the invention is that it includes a magnetically modifiedelectrode.

Another feature of the invention is that it includes a process forcoating a surface of a device with a magnetic composite materialresponsive to an external magnetic field, wherein the coating impartsincreased electrochemical efficiency, and/or increased flux, at saidsurface.

Another feature of the invention is that it includes magnetic compositesmade from ion exchange polymers and non-permanent magnet microbeads withmagnetic properties which are susceptible to externally applied magneticfields.

Another feature of the invention is that it includes magnetic compositesmade from ion exchange polymers and organo-Fe (superparamagnetic orferrofluid) or other permanent magnetic and nonpermanent magnetic orferromagnetic or ferrimagnetic material microbeads which exhibitmagnetic field gradients.

These and other objects, advantages and features are accomplished by aseparator arranged between a first region containing a first type ofparticle and a second type of particle and a second region, comprising:a first material having a first magnetism; a second material having asecond magnetism; a plurality of boundaries providing a path between thefirst region and the second region, each of the plurality of boundarieshaving a magnetic gradient within the path, the path having an averagewidth of approximately one nanometer to approximately severalmicrometers, wherein the first type of particles have a first magneticsusceptibility and the second type of particles have a second magneticsusceptibility, wherein the first and the second magneticsusceptibilities are sufficiently different that the first type ofparticles pass into the second region while most of the second type ofparticles remain in the first region.

These and other objects, advantages and features are also accomplishedby the provision of a cell, comprising: an electrolyte including a firsttype of particles; a first electrode arranged in the electrolyte; and asecond electrode arranged in the electrolyte wherein the first type ofparticles transform into a second type of particles once the first typeof particles reach the second electrode, the second electrode having asurface with a coating which includes: a first material having a firstmagnetism; a second material having a second magnetism; a plurality ofboundaries providing a path between the electrolyte and the surface ofthe second electrode, each of the plurality of boundaries having amagnetic gradient within the path, the path having an average width ofapproximately one nanometer to approximately several micrometers,wherein the first type of particles have a first magnetic susceptibilityand the second type of particles have a second magnetic susceptibility,and the first and the second magnetic susceptibilities are different.

These and other objects, advantages and features are also accomplishedby the provision of a method of coating a surface with a magneticcomposite material, comprising the steps of: preparing a casting mixturecomprising magnetic microbeads and an ion exchange polymer; casting thecasting mixture on the surface; and drying the casting mixture to yielda magnetic composite coating on the surface.

These and other objects, advantages and features are also accomplishedby the provision of a method of coating a surface of a device with amagnetic composite material, comprising the steps of: mixing a firstsuspension comprising at least about 1% by weight of inert polymercoated magnetic microbeads in a first solvent with a second suspensioncomprising at least about 2% by weight of ion exchange polymers in asecond solvent to yield a mixed suspension or casting mixture; applyingthe mixed suspension to the surface, the surface being arranged in amagnetic field of at least about 0.5 Tesla and being orientedapproximately 90° with respect to the normal of the electrode surface;and evaporating the first solvent and the second solvent to yield asurface coating having a plurality of boundary regions with magneticgradients having paths to the coated surface.

These and other objects, advantages and features are also accomplishedby the provision of a method of making an electrode with a surfacecoated with a magnetic composite with a plurality of boundary regionswith magnetic gradients having paths to the surface of the electrode,comprising the steps of: mixing a first component which includes asuspension of at least approximately 1 percent by weight of inertpolymer coated magnetic microbeads containing between approximately 10percent and approximately 90 percent magnetizable material havingdiameters at least 0.5 micrometers in a first solvent with a secondcomponent comprising at least approximately 2 percent by weight of anion exchange polymer in a second solvent to yield a casting mixture ormixed suspension; applying the mixed suspension to the surface of theelectrode, the electrode being arranged in a magnetic field of at leastapproximately 0.05 Tesla and being oriented approximately 90 degreeswith respect to the normal of the electrode surface; and evaporating thefirst solvent and the second solvent to yield the electrode with asurface coated with the magnetic composite having a plurality ofboundary regions with magnetic gradients having paths to the surface ofthe electrode.

These and other objects, advantages and features are furtheraccomplished by a method of making an electrode with a surface coatedwith a composite with a plurality of boundary regions with magneticgradients having paths to the surface of the electrode when an externalmagnetic field is turned on, comprising the steps of: mixing a firstcomponent which includes a suspension of at least 5 percent by weight ofinert polymer coated microbeads containing between 10 percent and 90percent magnetizable non-permanent magnetic material having diameters atleast 0.5 micrometers in a first solvent with a second componentcomprising at least 5 percent by weight of an ion exchange polymer in asecond solvent to yield a casting mixture or mixed suspension; applyingthe mixed suspension to the surface of the electrode; evaporating thefirst solvent and the second solvent to yield the electrode with asurface coated with the composite having a plurality of boundary regionswith magnetic gradients having paths to the surface of the electrodewhen an external magnet is turned on.

These and other objects, advantages and features are furtheraccomplished by a method of forming a graded density layer, comprisingthe steps of: formulating a series of casting mixtures, each of theseries of casting mixtures comprising a polymeric material and asolvent, wherein the series of casting mixtures have a range ofconcentrations of the polymeric material; applying to a surface a filmof one of the series of casting mixtures and evaporating the solvent;and, further applying to the surface a film of another of the series ofcasting mixtures and evaporating the solvent, to yield a coating of thegraded density layer on the surface.

These and other objects, advantages and features are also accomplishedby an electrode for channeling flux of magnetic species comprising: aconductor; a composite of a first material having a first magnetism anda second material having a second magnetism in surface contact with theconductor, wherein the composite comprises a plurality of boundariesproviding pathways between the first material and the second material,wherein the pathways channel the flux of the magnetic species throughthe pathways to the conductor.

These and other objects, advantages and features are furtheraccomplished by an electrode for effecting electrolysis of magneticspecies comprising: a conductor; and magnetic means in surface contactwith the conductor for enhancing the flux of the magnetic species in anelectrolyte solution to the conductor and thereby effecting electrolysisof the magnetic species.

These and other objects, advantages and features are furtheraccomplished by an electrode for effecting electrolysis of magneticspecies comprising: a conductor; and means in surface contact with theconductor for enhancing the flux of the magnetic species to theconductor and thereby effecting electrolysis of the magnetic species.

These and other objects, advantages and features are yet furtheraccomplished by an electrode for electrolysis of magnetic speciescomprising: a conductor; a composite magnetic material in surfacecontact with the conductor, the composite magnetic material having aplurality of transport pathways through the composite magnetic materialto enhance the passage of the magnetic species to the conductor andthereby effecting electrolysis of the magnetic species.

These and other objects, advantages and features are also accomplishedby a system, comprising: a first electrolyte species with a firstmagnetic susceptibility; a second electrolyte species with a secondmagnetic susceptibility; and a means for channeling the firstelectrolyte species with a first magnetic susceptibility preferentiallyover the second electrolyte species with a second magneticsusceptibility, wherein the means comprises a first material having afirst magnetism forming a composite with a second material having asecond magnetism.

These and other objects, advantages and features are also accomplishedby a system for separating first particles and second particles withdifferent magnetic susceptibilities comprising: a first magneticmaterial with a first magnetism; and a second magnetic material with asecond magnetism working in conjunction with the first magnetic materialto produce magnetic gradients, wherein the magnetic gradients separatethe first particles from the second particles.

These and other objects, advantages and features are accomplished by acomposite material for controlling chemical species transportcomprising: an ion exchanger and; a graded density layer, wherein theion exchanger is sorbed into the graded density layer. These and otherobjects, advantages and features are further accomplished by a magneticcomposite material for controlling magnetic chemical species transportaccording to magnetic susceptibility comprising: an ion exchanger; apolymer coated magnetic microbead material; and a graded density layer,wherein the ion exchanger and the polymer coated magnetic microbeadmaterial are sorbed into the graded density layer.

These and other objects, advantages and features are furtheraccomplished by a composite material for controlling chemical speciesviscous transport comprising: an ion exchanger; a graded viscositylayer, wherein the ion exchanger is sorbed into the graded viscositylayer.

These and other objects, advantages and features are furtheraccomplished by a magnetic composite material for controlling magneticchemical species transport and distribution comprising: an ionexchanger; a polymer coated magnetic microbead material; and a gradeddensity layer, wherein the ion exchanger and the polymer coated magneticmicrobead material are sorbed into the graded density layer forming agradient in the density of the polymer coated magnetic microbeadmaterial substantially perpendicular to a density gradient in the gradeddensity layer.

These and other objects, advantages and features are furtheraccomplished by a magnetic composite material for controlling magneticchemical species transport and distribution comprising: an ionexchanger; a polymer coated magnetic microbead material; and a gradeddensity layer, wherein the ion exchanger and the polymer coated magneticmicrobead material are sorbed into the graded density layer forming agradient in the density of the polymer coated magnetic microbeadmaterial substantially parallel to a density gradient in the gradeddensity layer.

These and other objects, advantages and features are also accomplishedby an ion exchange composite with graded transport and chemicalproperties controlling chemical species transport comprising: an ionexchanger; and a laminate graded density layer having a first side and asecond side, wherein the ion exchanger is one of sorbed into the gradeddensity layer and cocast on the graded density layer and the laminategraded density layer and the ion exchanger are contained within thefirst side and the second side, wherein the first side is in closerproximity to the source of the chemical species and the second side ismore distal to the source of the chemical species, and wherein thelaminate graded density layer has lower density toward the first sideand higher density toward the second side, substantially increasing indensity in a direction from the first side toward the second side.

These and other objects, advantages and features are also accomplishedby an ion exchange composite with graded transport and chemicalproperties controlling chemical species transport comprising: an ionexchanger; and a laminate graded density layer having a first side and asecond side, wherein the ion exchanger is one of sorbed into the gradeddensity layer and cocast on the graded density layer, and the ionexchanger and the laminate graded density layer are contained within thefirst side and the second side, wherein the first side is in closerproximity to the source of the chemical species and the second side ismore distal to the source of the chemical species, and wherein thelaminate graded density layer has higher density toward the first sideand lower density toward the second side, substantially decreasing indensity in a direction from the first side toward the second side.

These and other objects, advantages and features are accomplished alsoby a dual sensor for distinguishing between a paramagnetic species and adiamagnetic species comprising: a magnetically modified membrane sensor;and an unmodified membrane sensor, wherein the magnetically modifiedmembrane sensor preferentially enhances the concentration of and allowsthe detection of the paramagnetic species over the diamagnetic speciesand the unmodified membrane sensor enhances the concentration of andallows the detection of the diamagnetic species and the paramagneticspecies, enabling the measurement of the concentration of at least theparamagnetic species. These and other objects, advantages and featuresare further accomplished by a dual sensor for distinguishing between aparamagnetic species and a nonmagnetic species comprising: amagnetically modified membrane sensor; an unmodified membrane sensor,wherein the magnetically modified membrane sensor preferentiallyenhances the concentration of and allows the detection of theparamagnetic species over the nonmagnetic species and the unmodifiedmembrane sensor enhances the concentration of and allows the detectionof the nonmagnetic species and the paramagnetic species, enabling themeasurement of the concentration of at least the paramagnetic species.

These and other objects, advantages and features are furtheraccomplished by a dual sensor for distinguishing between a firstdiamagnetic species and a second diamagnetic species comprising: amagnetically modified membrane sensor; and a differently magneticallymodified membrane sensor; wherein the magnetically modified membranesensor preferentially enhances the concentration of and allows thedetection of the first diamagnetic species over the second diamagneticspecies and the differently magnetically modified membrane sensorenhances the concentration of and allows the detection of the secondparamagnetic species and the diamagnetic species, enabling themeasurement of the concentration of at least the first diamagneticspecies.

These and other objects, advantages and features are furtheraccomplished by a dual sensor for distinguishing between a firstparamagnetic species and a second paramagnetic species comprising: amagnetically modified membrane sensor; and a differently magneticallymodified membrane sensor, wherein the magnetically modified membranesensor preferentially enhances the concentration of and allows thedetection of the first paramagnetic species over the second paramagneticspecies and the differently magnetically modified membrane sensorenhances the concentration of and allows the detection of the secondparamagnetic species and the first paramagnetic species, enabling themeasurement of the concentration of at least the first paramagneticspecies.

These and other objects, advantages and features are furtheraccomplished by a dual sensor for distinguishing between a diamagneticspecies and a nonmagnetic species comprising: a magnetically modifiedmembrane sensor; and an unmodified membrane sensor, wherein themagnetically modified membrane sensor preferentially enhances theconcentration of and allows the detection of the diamagnetic speciesover the nonmagnetic species and the unmodified membrane sensor enhancesthe concentration of and allows the detection of the nonmagnetic speciesand the diamagnetic species, enabling the measurement of theconcentration of at least the diamagnetic species.

These and other objects, advantages and features are furtheraccomplished by a flux switch to regulate the flow of a redox speciescomprising: an electrode; a coating on the electrode, wherein thecoating is formed from a composite comprising: a magnetic microbeadmaterial with aligned surface magnetic field; an ion exchange polymer;and an electro-active polymer in which a first redox form isparamagnetic and a second redox form is diamagnetic, wherein the fluxswitch is actuated by electrolyzing the electro-active polymer from thefirst redox form ordered in the magnetic field established by thecoating to the second redox form disordered in the magnetic field.

These and other objects, advantages and features are also accomplishedby a flux switch to regulate the flow of a chemical species comprising:an electrode; and a coating on the electrode, wherein the coating isformed from a composite comprising: a non-permanent magnetic microbeadmaterial; an ion exchange polymer; and a polymer with magnetic materialcontained therein in which a first form is paramagnetic and a secondform is diamagnetic, wherein the flux switch is actuated by reversiblyconverting from the paramagnetic form to the diamagnetic form when anexternally applied magnetic field is turned on or off.

The above and other objects, advantages and features of the inventionwill become more apparent from the following description thereof takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the influence of electrode orientation and solvent motionon magnetohydrodynamic fluid motion for one geometry.

FIG. 2 shows the influence of electrode orientation and solvent motionon magnetohydrodynamic fluid motion for a second geometry.

FIG. 3 shows the influence of electrode orientation and solvent motionon magnetohydrodynamic fluid motion for a third geometry.

FIGS. 4A and 4B show plots of km values for neutron-track etchedpolycarbonate/Nafion composites versus functions of pore diameter, d.

FIGS. 5A and 5B show the surface diffusion model assuming no limitationsto the transport rate in the radial direction.

FIGS. 6A and 6B show the surface diffusion model including radialmigration.

FIGS. 7A and 7B show km values of hydroquinone throughpolystyrene/Nafion composites for ratios of surface area of themicrobeads to volume of Nafion.

FIG. 8 shows an analysis of fractal diffusion along the surface of themicrobeads in polystyrene Microbead/Nafion composites.

FIG. 9 shows preliminary km values for neutron-track etchedpolycarbonate/poly(4-vinylpyridine) composites.

FIG. 10 shows Kk values for Ru(NH₃)₆ ³⁺ as a function of volume fractionof microbeads in magnetic and nonmagnetic composites.

FIG. 11 shows the relative flux of redox species on the y-axis, wherethe maximum cyclic voltammetric current for a composite with magneticmicrobeads is normalized by the maximum cyclic voltammetric current fora Nafion film containing no magnetic material, with the ratio giving theflux enhancement.

FIGS. 12A, 12B, and 12C show cyclic voltammetric results for thereversible species Ru(NH₃)₆ ³⁺ and Ru(bpy)²⁺ and for the quasireversiblespecies hydroquinone.

FIG. 13 shows a plot of the flux for seven redox species that is usedfor predicting a roughly five-fold flux enhancement of oxygen through a15% magnetic Nafion composite over Nafion.

FIG. 14 shows a plot of the flux of Ru(NH₃)₆ ³⁺ in magnetic bead/Nafioncomposites increasing as the fraction of magnetic beads increases.

FIG. 15A shows a simplified representation used to describe how magneticmicroboundaries influence a standard electrochemical process.

FIG. 15B shows a simplified representation of embodiments of theinvention placed in an externally applied magnetic field provided by anelectromagnet to alter the magnetic properties of those embodiments,where the field may be turned on or off, or it may be oscillated.

FIG. 16 shows a simplified diagram of a separator with no electrode orconductive substrate which separates a mixture of particles between afirst solution and a second solution.

FIG. 17 is a short summary of steps involved in a method of making anelectrode according to two embodiments of the invention.

FIGS. 18A and 18B show a flux switch 800 to regulate the flow of a redoxspecies according to yet another embodiment of the invention.

FIG. 19 shows a dual sensor 900 for distinguishing between a firstspecies (particles A) and a second species (particles B).

FIG. 20 shows a cell 201 according to another embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Interfacial Gradients inGeneral

It has been found that interfacial gradients of concentration, charge,dielectric constant, and potential tend to establish strong, interfacialforces which decay over a microstructural distance (1 to 100 nm). (Forexample, for an applied potential of 10 mV to 100 mV past the potentialof zero charge at an electrode in 0.1 M aqueous electrolyte, theinterfacial potential gradient (|electric field|) is 10⁵ V/cm to 10⁶V/cm, but it decays over a distance of about 1 nm.) In a homogeneousmatrix, with few interfaces, interfacial gradients have a negligibleeffect on bulk material properties. However, in a microstructured matrixwhere the ratio of surface area to volume is high, interfacial gradientscan have a large effect on, or even dictate the properties of acomposite. Models appropriate to the description of bulk materials havebeen found to be unsatisfactory when applied to these composites.Moreover, such composites provide an opportunity to design matrices toperform functions and exhibit properties not found in homogeneousmaterials, as will be discussed.

The effects of gradients, associated with the interfaces between the ionexchanger and its support matrix, to enhance the transport of ions andmolecules have been studied in ion exchange polymer composites. Thecomposites were formed by sorbing ion exchange polymers into highsurface area substrates with well-established geometries. The flux ofsolutes through the composites was determined voltammetrically. When thesolute flux through the ion exchange portion of the composites and theflux through simple films of the ion exchanger were compared, fluxenhancements were observed. These enhancements were often greater thanan order of magnitude. Consistently, the ratio of surface area of thesubstrate to the volume of sorbed ion exchanger (SA/Vol) has been thecritical factor in quantifying the flux enhancements. The fluxenhancement characteristics were found to be dominated by the interfacebetween the ion exchanger and the support. Several interfacial gradientshave so far been identified as important: concentration gradients,leading to surface diffusion; electric potential gradients, leading tomigration; and magnetic field gradients, leading to flux enhancementsand electric potential shifts at electrodes.

Forming Composites

Composites were made by intimately mixing two or more components to forma heterogeneous matrix as will be discussed in more detail below. Whilecomposites retain some characteristics of their components, propertiesdistinct from those of the starting materials have been demonstratedthat make composites of special interest.

Results

The impact of microstructure on transport and selectivity in ionexchange polymers and their composites has been found to be significant.Novel characteristics arose not from the individual components of thecomposites, but from gradients established at the interfaces between thecomponents. Ion exchange polymers with inherent microstructure, such asNafion, exhibit superior transport, selectivity, and stabilitycharacteristics compared to polymers with no inherent microstructure,such as poly(styrene sulfonate). When ion exchange polymers weresupported on inert substrates with microstructural (5 to 100 nm)features similar in length scale to the microstructural features of theion exchanger (e.g., 5 nm micelles in Nafion), the structure of the ionexchanger was disrupted in an ordered manner. The relationship betweenthe flux characteristics of the composites and the microstructureimposed by the substrates has yielded information about howmicrostructure contributes to the properties of ion exchangers. Thisrelationship allows the specification of design paradigms for tailoringcomposites with specific transport and selectivity characteristics.

Surface Diffusion

The first composites studied in this regard were formed by sorbingNafion into the collinear cylindrical pores of neutron track etchedpolycarbonate membranes. The ion exchange polymer Nafion is aperfluorinated, sulfonic acid polymer with the following structure:

The SO³⁻ groups adsorb on the inert substrates to form a loosely packedmonolayer of perfluorinated alkyl chains, OCF₂CF₂OCF₂CF₂SO₃ ⁻, shownabove in boldface. This creates a unique interfacial zone approximately1 to 2 nm thick along the edge formed between the ion exchange polymerand the inert substrate. In systems with high surface area to volumeratio, a large fraction of the molecules and ions which passed throughthese composites actually moved through this interfacial zone. That is,it was found that the molecules and ions have higher flux in this thininterfacial zone, where the interfacial fields were strongest.

In a given membrane, all pores had approximately the same diameter, d,ranging between 15 and 600 nm. The flux of electro-active speciesthrough the composites was determined by rotating disk voltammetry. Inrotating disk voltammetry, the product km (cm²/s) parameterizes the fluxof a redox species through the Nafion portion of the composites, where kis the partition coefficient of the species into the Nafion and m(cm²/s) is its mass transport coefficient. Simple Nafion films castdirectly onto the electrode were also studied. The resulting plots of kmas a function of log(d) are shown in FIG. 4A. As indicated in FIG. 4A,as the pore diameter decreased towards 30 nm, the flux through theNafion portion increased as much as 3600% over the simple films. Thesestudies showed that the interface between Nafion and a support matrixwas pivotal in determining the flux characteristics of the composites.

The flux enhancement model proposed here depends on the interface formedbetween the Nafion and the polycarbonate providing a facile transportpathway to the electrode for the redox species. Bulk Nafion located inthe center of the pore had a smaller transport coefficient (m) than thesupport matrix wall, but provided a volume to extract redox species fromthe center of the pore to feed the wall transport zone. The criticalparameter for flux enhancement was found to be (for a cylindrical crosssection path) the ratio of the surface area of the wall providing faciletransport (πdλ), where λ is the layer thickness, to the volume of Nafionfeeding the interface (πd²λ/4), i.e., 4/d. Plots of km versus 1/d areshown in FIG. 4B. Note that the plots are linear in FIG. 4B for d≧30 nm,and with the exception of dopamine, the intercepts as d→∞(1/d→0)correspond to km for bulk Nafion.

Predictive models of how interfaces and their associated concentration,field, etc. gradients dictate interface properties and function areprovided below and further aid in the design of new composites tailoredfor specific applications. A simple surface diffusion model assuming nolimitations to the transport rate in the radial direction is outlined.FIGS. 5A and 5B show the simple model where transport in the radialdirection is not rate limiting. In the model, J_(comp) is the total fluxthrough the composites, J_(Nuc) is the flux through an empty pore, andJ_(bulk) and J_(wall) are the fluxes in the bulk (center) of the poreand along the surface of the pore, respectively. To analyze the flux, asin FIG. 4B, J_(bulk) and J_(wall) must be normalized to the crosssectional area of the pore used to determine km, the product of theeffective extraction and transport coefficients. From the finalequation, the plot in FIG. 4B can be interpreted to have the slope andintercept shown in FIGS. 5A and 5B. If δ, the thickness of theinterfacial zone, is taken as 1.5 nm, the values cited fork_(wall)m_(wall) and k_(bulk)m_(bulk) are found. The diffusioncoefficients of each species in solution are also listed for comparison.In general, K_(wall)m_(wall)˜(10 to 10²)Xk_(bulk)m_(bulk)˜(1 to10)xD_(soln). In other words, for an interfacial zone thickness, δ, of1.5 nm, k_(wall)m_(wall) is up to one order of magnitude higher thanD_(soln), and one to two orders of magnitude higher thank_(bulk)m_(bulk).

The interfacial transport zone occurs because of the irreversibleexchange of Nafion sulfonic acid groups to polycarbonate surface sitesto form a monolayer of inactive sulfonic acid groups. The side chainslinking the sulfonic acid sites to the Nafion backbone form a looselypacked monolayer along the pore wall which facilitates the flux throughthe transport zone compared to transport through the tortuousenvironment of bulk Nafion. Given the length of the chains, a δ value ofabout 1.5 nm is consistent with k_(wall)m_(wall) (and km/D_(soln))decreasing as transport is more hindered with increasing diameter of theredox species; i.e., k_(wall)m_(wall) decreases as H₂Q (0.6 nm)>Ru(NH₃)₆³⁺(0.8 nm)>DOP⁺(0.8 nm⁶)>FerN⁺(1 nm). Discrimination between thesespecies has also been observed based on molecular shape in the neutrontrack-etched composites. For example, disk shaped molecules exhibithigher flux than comparably sized spherical molecules.

Radial Migration

The pore walls have a surface charge density of −0.2 μC/cm². For a 30 nmpore diameter composite, the corresponding charge is 0.5% of the totalcharge in the pore, and will have negligible effect on the number ofcations extracted from the solution to move into the pore. However, thesurface charge establishes a potential gradient (electric field) fromthe pore to the wall which tends to move positively charged ionsradially outward from the center of the pore to the wall. An issue iswhether this radial, interfacial potential gradient can be coupled tothe concentration gradient along the wall to enhance solute flux to theelectrode, as illustrated in FIGS. 6A and 6B.

The model was tested by varying the concentration of the electrolyte,nitric acid, from 0.50 to 0.01 M, for fixed dopamine concentration (2mM). Flux was determined by rotating disk voltammetry at 400 rpm for thebare electrode and at infinite rotation rate for the modified electrodes(See Table 1). The electrolyte concentration did not dramatically affectthe flux for the bare electrode, the 30 nm membrane containing noNafion, and the Nafion film.

TABLE 1 Flux (nmol/cm²s) for Dopamine Oxidation at Various [H+] 400 rpmNafion Nafion Flux Flux Flux Flux [H+]_(soln) unmodified 30 nm Film 30nm 0.50M 38.6 54.8 4.2 2.4 0.10M 36.7 57.5 4.3 10.5 0.01M 44.6 73.1 —39.0

However, for the 30 nm Nafion composite a fifty-fold decrease inelectrolyte concentration led to >1600% increase in flux. Coupling ofradial flux, driven by the interfacial potential gradient, to surfacediffusion generates the enhancement. No enhancements were observed for asimilar study of neutral hydroquinone. It should be noted that onlycharged species move by migration; dopamine is charged, whilehydroquinone is not.

Since the selectivity coefficient for dopamine over protons is about 500in Nafion, decreasing the electrolyte concentration fifty-fold onlydecreases the dopamine concentration by 10%. The dramatic effectproduced by varying the proton concentration means that the protons, notthe dopamine, compensate the wall charge to establish the interfacialpotential gradient and enhance the radial flux of dopamine. This ispossible because the dopamine, a cationic amine, is heavily ion pairedto the sulfonic acid sites. With a dielectric constant of 20,substantial ion pairing can be anticipated in Nafion. Ion pairing mayexplain why the flux of cationic amines is lower than neutralhydroquinone as can be seen with reference to FIGS. 4A and 4B which showkm values for neutron-track etched polycarbonate/Nafion composites. FIG.4A shows km versus log(d), where d is the pore diameter. km increasesabove the values for bulk Nafion as d approaches 30 nm. Theconcentrations are 2 mM redox species and 0.1 M electrolyte forRuN+—Ruthenium (II) hexamine (□), H₂Q—Hydroquinone (Δ,∇), DOP+—Dopamine(∘), and FerN+—Trimethylamminomethyl ferrocene (⋄). The electrolyte isH₂SO₄ in all cases except for DOP+ and H2Q(∇). Lines represent no modeland are only intended to indicate the trend in the data. FIG. 4D showskm versus d⁻¹, where 4d⁻¹ is the surface area of the pore/volume ofNafion in the pore. As illustrated in FIGS. 6A and 6B, the slopes inFIG. 4B are indicative of the surface flux, and the interceptcorresponds to the flux in bulk Nafion. Note, all the redox speciesexcept hydroquinone are charged amines, and all have lower flux thanhydroquinone.

Vapor Phase Electrochemistry/Microstructure in Two-dimensions

One way to alter microstructure is to reduce the conduction matrix fromthree to two-dimensions. A two-dimensional system is made by sulfonatingthe nonionic, polymeric insulator between the electrodes of amicroelectrode assembly. Conduction across the surface cannot be studiedin either an electrolyte solution or a pure solvent as the liquidprovides a conductive path between the electrodes. However, bysupporting the microelectrode assembly in an evacuated flask, andinjecting hydrogen or hydrogen chloride and a small amount (:L) ofwater, conduction can be studied by electrolyzing the gas. In theselower dimensional systems, the role of the ion exchange site and itsconcentration, as well as the role of water in ionic conduction can bestudied. Preliminary studies were performed to study conduction throughsolvent layers adsorbed from the vapor phase across the nonionic surfaceof a microelectrode assembly. Electrolysis of gas phase solventsrequired the solvent to adsorb at greater than monolayer coverage tobridge the gap between the electrodes. Solvents with high autoprotolysisand acidity constants sustain higher currents than solvents less able togenerate ions. These studies provided information about gas phaseelectrochemical detection and systems as well as atmospheric corrosion.

Composites Formed with Polymerized Microspheres

To test the generality of flux enhancement by interfacial forces,composites of Nafion and polymerized polystyrene microbeads ormicrospheres were formed; diameters of 0.11 to 1.5 μm were used. FIGS.7A and 7B show km of hydroquinone through polystyrene microbead/Nafioncomposites versus ratios of surface area of the microbeads to volume ofNafion. In particular, values of Km found for various ratios of beadsurface area for transport to volume of Nafion for extraction (SA/Vol)are shown for three different bead diameters. As for the neutron tracketched composites, linear plots were found, at least for the largersizes, with intercepts comparable to bulk Nafion. Of these sizes, 0.37μm beads exhibited the largest flux enhancement (600%). FIG. 7A showsresults for composites formed with single size beads, where the ratio ofsurface area to volume was varied by varying the volume fraction ofbeads in the composites. Positive slopes are shown consistent with fluxenhancement by surface diffusion along the surface of the beads. Theintercepts are consistent with transport through bulk Nafion.

The fraction of microspheres in the composite can be varied anddifferent sizes mixed to allow a continuous range of SA/Vol. Inparticular, FIG. 7B shows results for composites for a range of SA/Volwith 50% total fraction of Nafion by volume in the film. km increases asSA/Vol increases to about 3.5×10⁵ cm⁻¹, analogous to 1.3×10⁶ cm⁻¹ foundfor the neutron track etched composites (FIG. 4A). Scanning electronmicrographs of the 50% Nafion, single bead size composites showedpacking of the 0.11 μm beads was different and may account for the lowerkm values found for d⁻¹>3.5×10⁵ cm⁻¹, where 0.11 μm beads were used.FIG. 7B shows results for composites formed with 50% Nafion by volume.The ratio of surface area to volume was varied by making composites withbeads of the same size and with beads of two different sizes. Fluxincreases as the ratio of surface area to volume increases to 3.5×10⁵cm⁻¹; at the highest ratio, the composite contains 0.11 μm beads.

From the scanning electron micrographs, composites of beads larger than0.11 μm exhibit the self-similarity typical of fractal materials. Whenln(km) for these beads is plotted versus log(d), where d is the beaddiameter, a linear plot with a slope of −0.733 was obtained; km versusd^(−0.733) is shown in FIG. 8. For diffusion on a fractal of finitelyramified structure (e.g., the Sierpinski gasket), this is the powerdependence expected for diffusion in a two-dimensional system. Thus,microbead composites exhibit transport typical of fractal diffusionalong the microbead surface. This system confirms that surface diffusionprovides a mechanism of flux enhancement. It also introduces the conceptof fractal transport processes and the importance of surfacedimensionality in ion exchange composites.

Poly(4-Vinylpyridine) Composites Formed on Neutron Track EtchedMembranes

To investigate surface diffusion in other ion exchangers, compositeswere formed of protonated poly(vinylpyridine (PVP)) and track etchedmembranes. From preliminary results, flux enhancements in thesecomposites increased with d (volume/surface area); see FIG. 9. Such adependency may be consistent with a transport rate which variesmonotonically in the radial coordinate. Physically, a non-uniformdensity of PVP, produced by interaction with the wall charge, couldgenerate a radially dependent transport rate.

Thermal Processing of Nafion

While commercial Nafion is heat cast, a process that yields invertedmicelles, the vast majority of academic studies of Nafion have beenperformed on cold cast Nafion which produces normal micelles. A study ofthe mechanical properties of Nafion hot cast from organic solvents hasbeen reported. Attempts have been made to hot cast Nafion films withmicrowave heating. In the highly ionic casting solution, the glasstransition temperature of Nafion (105° C.) should be reached as thewater evaporates. Plots of flux as a function of the time microwavedhave a break at approximately 15 minutes. The flux changed by no morethan a factor of three with a decrease in the flux of hydroquinone, andfrom preliminary studies, an increase in the flux of Ru(NH₃)₆ ³⁺. Thismay indicate different transport mechanisms for the two species in thefilm. Microwaved, cold cast and commercial hot cast films have beencompared.

Magnetic, Demagnetized, and Nonmagnetic Composites

Polystyrene coated, 1 to 2 μm Fe/Fe oxide (nonpermanent magneticmaterial) or organo-Fe (superparamagnetic or ferrofluid or permanentmagnetic) microbeads are available (Bangs Labs or Polyscience) as a 1%suspension in water, and Nafion (C.G. Processing) is available as a 5%suspension in alcohol/water (other inert or active polymer coatingsbesides polystyrene could be employed as well, and in non-aqueousenvironments, it is possible to eliminate the polymer coating completelyto provide uncoated microbeads if, for example, its purpose is normallyonly to prevent oxidation in an aqueous environment). This discussionholds for superparamagnetic or ferrofluid or permanent magnetic ornonpermanent magnetic or ferromagnetic or ferrimagnetic materialmicrobeads in general. This discussion also holds for other magnets andother magnetic materials which include, but are not limited to,superconductors, and magnetic materials based on rare earth metals suchas cobalt, copper, iron, samarium, cerium, aluminum and nickel, andother assorted metal oxides, and magnetic materials based on neodymium,e.g., magnequench, which contains iron and boron in addition toneodymium. The polymer coatings are required for use of these microbeadsin an aqueous environment to prevent oxidation, but in a nonaqueousenvironment the polymer coating may not be required. Magnetic compositesincorporating organo-Fe material microbeads are formed by castingappropriate volumes of each suspension onto an electrode centered insidea cylindrical magnet (5 cm inside diameter, 6.4 cm outside diameter, 3.2cm height; 8 lb pull). Once the solvents evaporate and the magnet isremoved, the oriented beads are trapped in the Nafion, stacked inpillars normal to the electrode surface. To minimize interbeadrepulsion, pillars form by stacking the north end of one bead to thesouth end of another; to minimize interpillar repulsion, the pillarsarrange in a roughly hexagonal array. These aligned composites wereformed with microbead fractions of ≦15%. Aligned composites werecompared to other composites: unaligned composites—formed as above butwith Fe/Fe oxide microbeads and without the magnet; nonmagneticcomposites—formed with 1.5 μm nonmagnetic polystyrene beads; simpleNafion films; and demagnetized composites—aligned composites that weredemagnetized. Demagnetized composites had the pillared structure, but itis not clear if they were fully demagnetized. Nonmagnetic composites hada coral-like structure (i.e., they do not form pillars). Note,composites may be formed wherein at least one component is reversiblychangeable between a paramagnetic form and a diamagnetic form with, forexample, a temperature variation with or without the presence of anexternally applied magnetic field.

Magnetic Composites Electrochemical Studies of Magnetic Composites

The composite was equilibrated in a solution of 1 mM electro-activespecies and 0.1 M electrolyte. The mass transport-limited current forthe electrolysis of the redox species through the composite (i_(meas))was then determined by steady-state rotating disk voltammetry at severaldifferent rotation rates (ω). A plot of i_(meas) ⁻¹ versus ω⁻¹ yielded aslope characteristic of transport in solution, and an interceptcharacteristic of transport through the composite as: $\begin{matrix}{\frac{n\quad F\quad A}{i_{meas}} = {{\frac{v^{1/6}}{0.62c*D_{soln}^{2/3}}\omega^{{- 1}/2}} + \frac{l}{K\quad m\quad {\varepsilon c}^{*}}}} & (3)\end{matrix}$

In Equation (3), n is the number of electrons, F is the Faradayconstant, A is the electrode area, c* and D_(soln−) are theconcentration and diffusion coefficient of the redox species insolution, respectively, v is the kinematic viscosity, l is the compositethickness, k is the partition coefficient of the redox species, m is themass transport rate of the redox species in the composite, and ∈ is theporosity of the composite. The partition coefficient, k, is the ratio ofthe equilibrium concentration in the ion exchange portion of thecomposite to the solution concentration, in the absence of electrolysis.Equation (3) is appropriate for rate-limiting transport perpendicular tothe electrode. This is ensured by choosing l and D^({fraction (1/3,)})_(solnω) ^(½v⅙) large compared to the microstructural dimensions of thecomposite, and is verified by the slope. Then, the composite can betreated as homogeneous with an effective km, and microstructural effectscan be ascertained with rotating disk studies. Cyclic voltammetryyielded quantitative information for scan rates, v, sufficient tocontain the transport length within the composite. For a reversiblecouple, the peak current, i_(peak), is

i _(peak)=0.4463(nF)^(½) [v/RT] ^(½),   (4)

where R is the gas constant and T is the temperature. When both rotatingdisk and cyclic voltammetry data are obtainable, k and m are separablebecause of their different power dependencies in Equations (3) and (4).

The flux of redox species through magnetic composites is enhanced inproportion to the absolute value of the difference in the magneticsusceptibilities of the products and reactants of the electrolysis. Fromcyclic voltammetry, the ΔE_(p) observed for reversible species, whetherparamagnetic or diamagnetic, was little changed, but E_(0.5) wasshifted, where E_(0.5) is the average of the anodic and cathodic peakpotentials, and is a rough measure of the free energy of the electrontransfer reaction. For a quasireversible, diamagnetic species whichpassed through a radical intermediate, dramatic changes in ΔE_(p) werefound. The shifts and peak splittings were consistent with thestabilization and the concentration of the paramagnetic species. Resultsare summarized below.

Flux Enhancements for Paramagnetic Species

Values of km found by rotating disk voltammetry for diamagnetichydroquinone and Ru(bpy)₃ ²⁺, and paramagnetic Ru(NH₃)₆ ³⁺ using Nafionfilms, nonmagnetic polystyrene microbead composites, and magneticmicrobead composites are summarized in Table 2. Both bead compositescontained 15% beads of 1 to 2 μm diameter; all modifying layers were 3.6to 3.8 μm thick.

TABLE 2 km (10⁻⁶cm²/s) for Various Magnetic/Nonmagnetic Species andFilms km_(Nafion) km km film Nonmagnetic Magnetic Hydroquinone 0.9251.02 2.21 Ru (bpy)₃ ²⁺ 0.290 0.668 0.869 Ru (NH₃)₆ ³⁺ 0.570 1.01 3.80

In these examples, as in general, when flux of redox species through themagnetic composite was compared to flux through either Nafion films orcomposites formed with nonmagnetic beads, the flux was enhanced. Ingeneral, we find the flux enhancement is not dependent on whether theelectrolysis is converting a diamagnetic to a paramagnetic species or aparamagnetic to a diamagnetic species, but that the enhancementincreases as the absolute value of the difference in the molar magneticsusceptibilities of the product and reactant.

To further investigate paramagnetic (Ru(NH₃)₆ ³⁺, km values were foundfor magnetic and nonmagnetic composites made with various fractions ofbeads. Results are shown in FIG. 10. First, in FIG. 10 the flux ofRu(NH₃)₆ ³⁺ increased strongly with the fraction of magnetic beads, butnot with the fraction of nonmagnetic beads. Second, since theenhancement is not linear with the magnetic bead fraction, theenhancement was not due to either a simple concentration increase of theparamagnetic species about each bead or a simple increase in surfacediffusion associated with more pillars at higher bead concentration.(Data are equally well linearized with correlation coefficient >0.99 aseither In[km] versus percent beads, or km versus volume ofNafion/surface area of the beads. Plots of both showed interceptscomparable to km for simple Nafion films.) Third, substantially higherflux was achieved with the magnetic beads than with the same fraction ofnonmagnetic beads.

Magnetohydrodynamic models neither account for the discriminationbetween paramagnetic and diamagnetic species by the magnetic composites,nor do they predict the shape of the curve shown in FIG. 10.

Electrochemical flux of various redox species from solution througheither composites or films to the electrode surface was determined bycyclic and steady-state rotating disk voltammetry. Electrochemical fluxof species through the composites is parameterized by k and m, where kis the extraction coefficient of the redox species from solution intothe composite, and m (cm²/s) is its effective diffusion coefficient. Forsteady-state rotating disk voltammetry, the parameterization is km(determined from the intercept of a Koutecky Levich plot), and forcyclic voltammetry, the parameterization is km^(½) (extracted from theslope of peak current versus the square root of the scan rate (20 to 200mV/s)). All measurements were made in solutions containing 1 to 2 mMredox species at a 0.45 cm² glassy carbon electrode. The electrolyte was0.1 M HNO₃, except for the reduction of Co(bpy)₃ ²⁺ (0.2 M Na₂SO₄) andfor the oxidation of Co(bpy)₃ ²⁺ and reduction of Co(bpy)₃ ³⁺ (0.1 Msodium acetate/acetic acid buffer at pH=4.5). Anionic ferricyanide wasnot detected electrochemically through the anionic Nafion films andcomposites, consistent with defect-free layers. All potentials wererecorded versus SCE.

First, km values were determined for the oxidation of paramagneticRu(NH₃)₆ ³⁺ to diamagnetic Ru(NH₃)₆ ²⁺ through magnetic and nonmagneticcomposites as the bead fraction was increased. |Δχ_(m)|=1,880×10⁻⁶cm³/moles. From FIG. 10, km for the nonmagnetic composites varies littlewith bead fraction, while km for the magnetic composites increasessuperlinearly by several fold.

Second, km^(½) values were determined for various redox reactions formagnetic composites, nonmagnetic composites, and Nafion films. Exclusiveof any magnetic field effects, electrochemical flux through Nafion canbe altered by the size, charge, and hydrophobicity of the transportedspecies, interaction and binding with the exchange sites, andintercalation into the hydrated and perfluorinated zones of the Nafion.To minimize effects not related to interactions between the redoxmoieties and the magnetic beads, km^(½) values for the magnetic andnonmagnetic composites are normalized by km^(½) for the Nafion films.The normalized km^(½) values are plotted in FIG. 11 versus |Δχ_(m)| forthe various redox reactions. FIG. 11 illustrates the relative flux ofredox species on the y-axis, where the maximum cyclic voltammetriccurrent for a composite with magnetic microbeads is normalized by themaximum cyclic voltammetric current for a Nafion film containing nomagnetic material. The ratio is the flux enhancement. On the x-axis isthe absolute value of the difference in the molar magneticsusceptibilities of the products and reactants of the electrolysis,|Δχ_(m)|. The composites contain 15% magnetic microbeads and 85% Nafionby volume. The redox species are numbered as follows, where the reactantproducts are listed sequentially: (1) hydroquinone to benzoquinone; (2)Cr(bpy)₃ ³⁺ to Cr(bpy)₃ ²⁺; (3) Ru(bpy)₃ ²⁺ to Ru(bpy)₃ ³⁺; (4) Ru(NH₃)₆³⁺ to Ru(NH₃₎ ₆ ²⁺; (5) Co(bpy)₃ ²⁺ to Co(bpy)₃ ¹⁺; (6) Co(bpy)₃ ²⁺ toCo(bpy)₃ ³⁺; and (7) Co(bpy)₃ ³⁺ to Co(bpy)₃ ²⁺. All redox species are 1mM to 2 mM. Film thicknesses are 3.6 micrometers to 3.8 micrometers. Forthe nonmagnetic composites, the normalized km^(½) values are independentof |Δχ_(m)|. This suggests the normalization is effective in minimizingsteric and electrostatic differences in the interactions of the variousredox species with Nafion. For the magnetic composites, normalizedkm^(½) increases monotonically with |Δχ_(m)|, with the largestenhancements approaching 2000%.

The logarithmic increase of electrochemical flux in FIG. 11 with|Δχ_(m)| is consistent with a free energy effect of a few kJ/mole.Effects of this magnitude have not been generated in uniform,macroscopic magnetic fields. Strong, non-uniform magnetic fieldsestablished over short distances (a few nanometers) at the interfacebetween Nafion and magnetic microbeads could produce local effects ofthis magnitude. Magnetic concepts appropriate to uniform macroscopicmagnetic fields and to molecular magnetic interactions are notapplicable to this system, and instead, a microscopic parameterizationis necessary. Establishing sufficiently strong and nonuniform localmagnetic fields at interfaces in microstructured systems makes itpossible to orchestrate chemical effects in micro-environments whichcannot otherwise be achieved with uniform fields applied by largeexternal magnets.

Cyclic Voltammetric Peak Splittings for Quasireversible Species

Peak splittings in cyclic voltammetry are used to determinedheterogeneous electron transfer rates. FIGS. 12A and 12B show cyclicvoltammetric results for the reversible species Ru(NH₃)₆ ³⁺ and Ru(bpy)₃²⁺, respectively. Cyclic voltammograms at 100 mV/s are shown forRu(NH₃)₆ ³⁺ (FIG. 12A) and Ru(bpy)₃ ²⁺ (FIG. 12B) for magneticcomposites, Nafion films, and the bare electrode. Cyclic voltammetricresults are shown for the reduction of paramagnetic Ru(NH₃)₆ ³⁺ in FIG.12A. The concentration of the redox species is 1 mM, and the electrolyteis 0.1 M HNO₃; the reference is an SCE; and the films are 3.6 μm thick.For both species, when E_(0.5) is compared for the magnetic compositeand the Nafion films, the shift in E_(0.5) is to positive potentials.The electron transfer kinetics for Ru(NH₃)₆ ³⁺ are fairly strong withk⁰>0.2 cm/s. Note that the peak splittings for the magnetic compositesand Nafion film are similar, consistent with the resistance of the twolayers being similar. Similar peak splittings are also observed forRu(bpy)₃ ²⁺, as shown in FIG. 12B. Therefore, when compared to theNafion films, the magnetic composites have little effect on the rate ofelectron transfer of reversible species.

In particular, FIG. 12C shows cyclic voltammograms at 100 mV/s for 1 mMhydroquinone in 0.1 M HNO₃ for magnetic composites, nonmagneticcomposites, Nafion films, and the bare electrode. The films are 3.6 μmthick. It is observed in the voltammogram of FIG. 12C that the peaksplitting is almost doubled for the magnetic composite compared to theNafion film. The question arises as to whether the enhanced peaksplitting is consistent with the stabilization of the paramagneticsemiquinone intermediate in the two electron/two proton oxidation. InFIG. 12C, voltammograms are shown at 0.1 V/s for hydroquinone, adiamagnetic species that undergoes quasireversible, two electron/twoproton oxidation to diamagnetic benzoquinone while passing through aradical, semiquinone intermediate. The voltammograms for the Nafion filmand the nonmagnetic composites are fairly similar, with ΔE_(p) values of218 and 282 mV, respectively. For the magnetic composite, ΔE_(p)=432 mV,or twice that of the Nafion film. From the results for the reversiblecouples above, this is not due to a higher resistance in the magneticcomposites. The asymmetry in the peak shifts compared to the other threesystems shown in FIG. 12C also argues against a resistance effect. (Notethat the interpretation of the kinetics can be complicated by the protonconcentration. However, there is no reason to think the concentration isdrastically different in the magnetic and nonmagnetic composites.) Thepeak shift may be due to the stabilization of the paramagneticsemiquinone intermediate.

While the hydroquinone electrolysis is too complex to interpret cleanly,it does raise the interesting question of whether quasireversibleelectron transfer rates can be influenced by an applied magnetic field.Reversible rates will not be affected, but it is not clear what wouldhappen with quasireversible rates. There are many quasireversibleelectron transfer species uncomplicated by homogeneous kinetics anddisproportionation reactions which can be used to better resolve thisquestion. If the kinetics of quasireversible processes can be influencedby magnetic fields, numerous technological systems could be improved.

Cyclic Voltammetric Peak Shifts

When magnetic composites and Nafion films were compared, voltammogramstaken at 0.1 V/s for the reversible species exhibited no change inΔE_(p). However, the peak potential for reduction, E_(p) ^(red), forRu(NH₃)₆ ³⁺ was shifted 14 mV positive. Similarly, the oxidationpotential peak, E_(p) ^(ox), for Ru(bpy)₃ ²⁺ was shifted 64 mV positive.Shifts of E_(0.5) while ΔE_(p) is unchanged are consistent with onespecies being held more tightly in the composites, and thereby, having alower diffusion coefficient. In general, a shift in potential ofapproximately +35 mV is observed for all reversible redox species,whether the electron transfer process converts the redox species fromdiamagnetic to paramagnetic or paramagnetic to diamagnetic. Largerpotential shifts are observed with less reversible electron transferprocesses. Shifts as large as 100 mV have been observed. (Note that forthe film thicknesses used herein (˜3.6 μm) and a scan rate of 0.1 V/s,m≦10⁻⁸ cm²/s is needed for the diffusion length to be confined withinthe film during the sweep. Since m is not known in these systems, it isnot clear whether the voltammetric results also probe behavior at thecomposite/solution interface.)

The above discussion further shows that interfacial gradients other thanconcentration and electric potential, e.g., magnetic gradients, can beexploited effectively in microstructured matrices. In composites formedwith magnetic materials, locally strong (and nonuniform) magnetic fieldscould alter transport and kinetics. The influence of the magnetic fieldon species in composites may be substantial because the species areconcentrated in a micro-environment, where the distance between thefield source and chemical species is not large compared to the fielddecay length. Magnetic composites were made by casting films ofpolystyrene coated magnetic beads and the perfluorinated, cationexchange polymer, Nafion, onto an electrode. Approximately 1 μm diametermagnetic beads were aligned by an external magnet as the castingsolvents evaporated. Once the solvents evaporated and the externalmagnet was removed, the beads were trapped in the Nafion, stacked asmagnetic pillars perpendicular to the electrode surface.

Preliminary voltammetric studies comparing the magnetic composites tosimple Nafion films yielded several interesting results. First, flux ofredox species through magnetic microbead composites is enhanced comparedto flux through both simple Nafion films and composites formed withnonmagnetic microbeads. Second, for species which underwent reversibleelectron transfer (i.e., Ru(NH₃)₆ ³⁺ and Ru(bpy)₃ ²⁺) the cyclicvoltammetric peak potential difference (ΔE_(p)) was unaffected, but theaverage of the peak potentials (E_(0.5)) shifted consistent with thestabilization of the paramagnetic species. Third, hydroquinone oxidationwas quasireversible and proceeded through paramagnetic semiquinone. Forhydroquinone at 0.1 V/s, voltammograms for the magnetic compositesexhibited a 40 mV positive shift of E_(0.5) and a ΔE_(p) twice that ofNafion. The potential shifts and flux enhancements, while consistentwith concentration and stabilization of the paramagnetic form of theredox couples, are as yet unexplained.

Electrochemical flux of ions and molecules through magnetic compositesformed of Nafion ion exchange and polystyrene coated Fe/Fe oxideparticles has been observed to be as much as twenty-fold higher than theflux through simple Nafion films. Flux enhancements have been observedwith increasing difference in the magnetic susceptibility of the halvesof the redox reaction.

A passive, magnetic composite may be used to enhance the flux of oxygenat the cathode in a fuel cell. Oxygen has two unpaired electrons, and istherefore susceptible to this magnetic field in the same way asdescribed in the experiments above. If oxygen is consistent with theobservations made thus far for other ions and molecules, theelectrochemical flux of oxygen to a magnetically modified cathode can beenhanced by approximately 500% as compared to a nonmagnetic cathode(FIG. 13). Such an enhancement would be comparable to that achieved bypressurization to 5 atmospheres at the cathode.

Based on the above discussion, it is possible to predict a roughlyfive-fold flux enhancement of oxygen through a 15% magnetic/Nafioncomposite over Nafion. This is understood by considering the fluxesthrough magnetic/Nafion composites and Nafion films of the seven redoxspecies listed in the upper left hand corner of FIG. 13 and are the samespecies as listed in FIG. 11. The fluxes were determined by cyclicvoltammetry. The flux ratio for magnetic composites to Nafion films isthe y-axis and the absolute value of the difference in the molarmagnetic susceptibilities (|Δχ_(m)|) of products and reactants of theelectrolysis reaction is the x-axis of FIG. 13, respectively. (Thelarger the value of Δχ_(m), the more susceptible a species is tointeraction with a magnetic field.) From FIG. 13, the flux increasesexponentially as |Δχ_(m)| increases. For the most extreme case, the fluxis increased about twenty-fold. For the reduction of oxygen to water,|Δχ_(m)|≈3500×10⁻⁶ cm³/mole. This point on the x-axis is extrapolated totherefore suggest that the flux enhancement for oxygen in the magneticcomposite will approach five-fold.

Experiments have been conducted with Nafion composites of up to 15%Fe/Fe oxide particle beads. FIG. 14 shows a curve of the increase influx based on the percentage of magnetic beads. The dotted line on FIG.14 is the projected effect on flux of higher bead concentrations.

For paramagnetic species, the flux through the magnetic compositesincreases as the fraction of magnetic beads increases. In FIG. 14, theflux of Ru(NH₃)₆ ³⁺ through magnetic bead/Nafion composites ()increases as the fraction of magnetic beads in the composite isincreased to 15%. Larger enhancements may be possible with higher beadfraction composites or composites formed with magnetic beads containingmore magnetic material. Compared to a simple Nafion film (□), the fluxis 4.4 fold larger. Ru(NH₃)₆ ³⁺ is less paramagnetic than oxygen. Forcomparison, composites formed with nonmagnetic polystyrene beads (◯)were examined; these exhibited no flux enhancement as the bead fractionincreased. The line shown on the plot is generated as a logarithmic fitto the data for the magnetic composites. It illustrates the fluxenhancement that might be found for composites formed with a higherfraction of magnetic beads. The extrapolation suggests that at 30%magnetic beads, the flux through the magnetic composites of Ru(NH₃)₆ ³⁺might approach twenty times its value in simple Nafion films. As oxygenis more paramagnetic than Ru(NH₃)₆ ³⁺ even larger enhancements might beanticipated for oxygen.

Oxygen Susceptibility to Magnetic Composites and Magnetic Concepts

Paramagnetic molecules have unpaired electrons and are drawn into(aligned by) a magnetic field (i.e., a torque will be produced; if amagnetic field gradient exists, magnetic dipoles will experience a netforce). Radicals and oxygen are paramagnetic. Diamagnetic species, withall electrons paired, are slightly repelled by the field; most organicmolecules are diamagnetic. (Metal ions and transition metal complexesare either paramagnetic or diamagnetic.) How strongly a molecular orchemical species responds to a magnetic field ig parameterized by themolar magnetic susceptibility, χ_(m) (cm³/mole). For diamagneticspecies, χ_(m) is between (−1 to −500)×10⁻⁶ cm³/mole, and is temperatureindependent. For paramagnetic species, χ_(m) ranges from 0 to +0.01cm³/mole, and, once corrected for its usually small diamagneticcomponent, varies inversely with temperature (Curie's Law). While ionsare monopoles and will either move with or against an electric field,depending on the sign of the potential gradient (electric field),paramagnetic species are dipoles and will always be drawn into (alignedin) a magnetic field, independent of the direction of the magneticvector. A net force on a magnetic dipole will exist if there is amagnetic field gradient. The magnetic susceptibilities of speciesrelevant to this proposal are summarized below.

TABLE 3 Molar Magnetic Susceptibilities, χ_(m) Temperature Species (°K.) χ_(m)(10⁻⁶cm³/mole) O₂ 293 3449 H₂O 293 −13 H₂O₂ — −18

Magnetic field effects were observed in electrochemical systems. Becauseelectrochemistry tends to involve single electron transfer events, themajority of electrochemical reactions should result in a net change inthe magnetic susceptibility of species near the electrode. Little hasbeen reported, however, in electrochemistry on magnetic fields. What hasbeen reported relates to magnetohydrodynamics. Magnetohydrodynamicsdescribes the motion of the charged species (i.e., an ion) perpendicularto the applied magnetic field and parallel to the applied electric field(Lorentz force). In the composites described herein, the magnetic field,the direction of motion, and the electric field were all normal to theelectrode. Because magnetohydrodynamics (see FIGS. 1-3) does not predicta motion dependence on the magnetic susceptibility of the moving speciesand requires that all the field and motion vectors are perpendicular(i.e., for magnetic effects), the effects described here are unlikely tobe macroscopic magnetohydrodynamic effects.

Graded Density Composites

The following protocol is used to form graded density layers onelectrodes with the density layers parallel to the electrode surface orother surface: Solutions of Ficoll (a commercially available copolymerof sucrose and epichlorohydrin used to make macroscopic graded densitycolumns for separations of biological cells by their buoyancy) are madein water at concentrations varying from a few percent to 50% by weight.The viscosity of the solution is a monotonic function of the weightpercent polymer. Small volumes of polymer solution (5 to 100microliters) are pipetted onto an electrode surface and the electrodespun at 400 rpm for two minutes; this creates a single polymer layer. Byrepeating this process with polymer solutions of differentconcentrations, a graded interface with density and viscosity varied asa function of the composition of the casting solution can be created inthe form of a discontinuous density gradient and/or a discontinuousviscosity gradient. The thickness of each step in the resultantmulti-layered or laminate structure depends on the number of layers castof a given concentration, and can range from 200 nm to severalmicrometers.

A similar structure with graded layers of ion exchange sites in ionexchange polymers can be formed by either (1) spin casting or coating amixture of density gradient polymer and ion exchange polymer on theelectrode or other surfaces as described above; (2) forming a densitygraded layer of density gradient polymer first, and then adsorbing theion exchange polymer into the matrix or; (3) spin casting or coatinglayers of ion exchange polymer on surfaces from solutions or suspensionsof different concentrations. It should be possible to cast such layers,and then peel them off surfaces to form free standing films. Such filmswould have utility in controlling solvent transport acrosselectrochemical cells, including fuel cells.

A protocol is proposed to form graded density layers on electrodes withthe density layers perpendicular to the electrode surface or othersurface. Electrodes and surfaces can be envisioned in which more thanone gradient is established on the surface for purposes of separatingmolecules in more than one spatial and temporal coordinate and by morethan one property. One example is to form composites with a magneticgradient in one coordinate and a density gradient in the other. Thesematerials could be formed by creating a magnetic gradient perpendicularto the electrode surface by placing magnetic beads on an electrode orsurface and allowing the composite to be cast in a nonuniform field,where the external magnet is aligned so the beads are on the surface butnot in columns perpendicular to the surface. A density layer could becast (as opposed to spun cast or coated) by pipetting small volumes ofdifferent concentrations of density gradient polymer and/or ion exchangepolymer and allowing the solvents to evaporate, thereby building up agraded layer parallel to the electrode surface. Once the entire layer iscast, the external magnet can be removed if the magnetic material issuperparamagnetic, and left in place if the magnetic material isparamagnetic.

These would be fairly sophisticated composites, and complex tounderstand, but unusual flux enhancements and separations should bepossible in several dimensions. It should be possible to design evenmore complex structures than these.

Modified Ion Exchangers

The surface of the magnetic microbeads have ion exchange groups on themwhich would allow ready chemical modification, e.g., coating with amagnetically oriented liquid crystal for a local flux switch. Examplesof such modified structures may have use in the quest to buildmicrostructured devices and machines.

APPLICATIONS General Applications

FIG. 15A shows a simplified representation which will be used todescribe how magnetic gradients at boundary regions 33 influence astandard electrochemical process. Here, a substrate 20 with a surface 24serves as a conductor and hence can electrically conduct like a metal, asemiconductor or a superconductor. Substrate 20 is maintained at a firstpotential V1. Two different phases of materials 30 a and 30 b have twodifferent magnetic fields, i.e., are in two different magnetic phases,phase 1 and phase 2 and are applied to surface 24 of substrate 20. Sincematerials 30 a and 30 b have different magnetic fields, boundary regionsor boundaries 33 have magnetic gradients. Boundary regions 33 are notnecessarily sharp or straight, but the magnetic field of material 30 asmoothly changes into the magnetic field of material 30 b according toelectromagnetic boundary conditions. Therefore, width t represents anaverage width of boundaries 33. Width t should be approximately betweena one nanometer to a few micrometers and preferably between onenanometer and approximately 0.5 micrometers. Boundary regions 33 areseparated from each other by varying distances and S represents theaverage of these distances. The effect of varying distances S will bedescribed below.

Particles M have a magnetic susceptibility χ_(m) and are in anelectrolyte 40 which is at a potential V2 due to an electrode 50. Thismakes a potential difference of V between electrolyte 40 and substrate20 (substrate 20 can effectively act as a second electrode). Boundaryregions 33 are paths which can pass particles M. Particles M are theneither driven electrically or via a concentration gradient towardsubstrate 20. Once particles M reach substrate 20, they either acquireor lose electrons, thereby turning into particles N with magneticsusceptibility χ_(n). The absolute value of the difference between themagnetic susceptibilities of phase 1 and phase 2 is a measure of themagnitude of the magnetic gradient in region 33 and will be referred toas the magnetic gradient of boundary region 33. It will be shown belowthat the flux of particles M increases approximately exponentially withrespect to increasing the magnetic gradient of boundary region 33 withmaterials 30 a and 30 b when compared to the flux without materials 30 aand 30 b. This increase in flux can be over a factor of 35 or 3500%resulting in significant improvements in efficiency of manyelectrochemical processes.

Specific examples of electrochemical systems where magnets might improvean electrochemical cell or process include: chloralkali processing,electrofluoridation, corrosion inhibition, solar and photocells ofvarious types, and acceleration of electrochemical reactions at theelectrode and in the composite matrix. Potential shifts of E_(0.5) arealways observed and suggest an energy difference is generated by themagnetic fields and gradients in the composites; generically, this couldimprove performance of all electrochemical energy devices, includingfuel cells, batteries, solar and photocells. Other applications includesensors, including dual sensors for parametric species; optical sensors;flux switching; and controlled release of materials by control of amagnetic field, including release of drugs and biomaterials. There mayalso be applications in resonance imaging technology.

Boundaries 33 do not have to be equally spaced and do not have to haveequal widths or thicknesses t. Materials 30 a and 30 b can be liquid,solid, gas or plasma. The only restriction is that a boundary 33 mustexist, i.e., materials 30 a and 30 b must have two different magneticfields to create the magnetic gradient within the width t. The magneticgradient of region 33 can be increased by (1) increasing the magneticcontent of the microbeads; (2) increasing the bead fraction in thecomposite; (3) increasing the magnetic strength of the beads byimproving the magnetic material in the beads; and (4) enhancing thefield in the magnetic microbeads by means of an external magnet. Ingeneral, the flux of particles M and N is correlated with magneticsusceptibility properties, χ_(m) and χ_(n). The above phenomena can beused to improve performance of fuel cells and batteries.

FIG. 15B shows apparatus 80 which corresponds to any of the abovediscussed embodiments as well as the embodiments shown in FIG. 16 orthereafter. Some of the embodiments in their implementation require thepresence of a magnetic field such as that produced by electromagnet 70and some of the embodiments do not require electromagnet 70, althoughthey can do so. Apparatus 80 corresponds to, for example, someembodiments of the magnetically modified electrode, the fuel cell, thebattery, the membrane sensor, the dual sensor, and the flux switch.Electromagnet 70 can be any source of a magnetic field. Electromagnet 70can also be used in the above discussed methods of forming the compositemagnetic materials that require the presence of an externally appliedmagnetic field. Electromagnet 70 can be controlled by controller 72 toproduce a constant or oscillating magnetic field with power supplied bypower supply 74.

FIG. 16 shows another simplified diagram showing a second manifestationof the above described phenomenon and hence a second broad area ofapplication. Namely, FIG. 16 shows a separator 60 arranged between afirst solution 62 a and a second solution 62 b, separator 60 includesfirst and second materials 30 a and 30 b having two different magneticfields; materials 30 a and 30 b have boundaries 33 there between. Here,there is no electrode or conductive substrate 20. Solution 62 a has atleast two different types of particles M₁ and M₂ with two differentmagnetic susceptibilities χ_(m1) and χ_(m2), respectively. Onceparticles M₁ or M₂ drift into an area near any one of boundaries 33,they are accelerated through the boundaries 33 by the magnetic gradienttherein. Here, χ_(m1) is greater than χ_(m2), which causes the flux ofparticles M₁ through separator 60 to be greater than the flux ofparticles M₂ through separator 60. This difference in flux can again beover a factor of 35 or 3500%, and may somewhat cancel out any differencein acceleration due to different masses of particles M₁ and M₂.Consequently, if the above process is allowed to proceed long enough,most of the particles M₁ will have passed through separator 60 beforeparticles M₂, thereby making first solution 62 a primarily made up ofparticles M₂ and second solution 62 b primarily made up of particles M₁.Note, separation of particles M₁ and M₂ may require some specialtailoring of the separator 60 and also relies on how much time isallowed for particles M₁ and M₂ to separate. In an infinite amount oftime, both particles M₁ and M₂ may cross separator 60. Particle size mayalso have a bearing on the ultimate separation of particles M₁ and M₂ byseparator 60.

The above discussion with respect to FIG. 16 involves two types ofparticles, M₁ and M₂, but the discussion also holds for any number ofparticles. Consider, for example, solution 62 a having particles M₁, M₂,M₃ and M₄ with susceptibilities χ_(m1), χ_(m2), χ_(m3) and χ_(m4),respectively. If χ_(m1)>χ_(m2)>χ_(m3)>χ_(m4), then M₁ would pass moreeasily through separator 60, followed by M₂, M₃, and M₄. The greater thedifference between magnetic susceptibilities, the better the separation.The above phenomenon can be used to improve performance of fuel cellsand batteries. Other applications include separation technology ingeneral, chromatographic processes—including separation of highertransition metal species (lanthanides and actinides), and photography.

In the above discussion with respect to FIGS. 15 and 16, the greater thenumber of boundary regions 33 per unit area (i.e., the smaller S), thegreater the effects due to the presence of boundary regions 33macroscopically manifest themselves. S can vary from fractions of amicrometer to hundreds of micrometers. In quantum systems with smallerstructures, S is further reduced to less than approximately 10 nm.

Design paradigms are summarized below to aid in tailoring composites forspecific transport and selectivity functions.

Forces and gradients associated with interfaces, which are of noconsequence in bulk materials, can contribute to and even dominate thetransport processes in composites.

Increasing the microstructure of composites can enhance the influence ofinterfacial gradients.

The closer a molecule or ion is placed to the interface, the strongerthe effect of the interfacial field on the chemical moiety. Systemsshould be designed to concentrate molecules and ions near interfaces.

The ratio of surface area for transport to volume for extractionparameterizes surface transport.

Fields in a microstructural environment can be non-uniform, but locallystrong.

Strong but short range electrostatic and magnetic fields are betterexploited in microstructured environments than in systems withexternally applied, homogeneous fields.

Vectorial transport is plumbed into microstructured matrices by couplingtwo or more field or concentration interfacial gradients; the largesteffects will occur when the gradients are either perpendicular orparallel to each other.

Control of surface dimensionality (fractality) is critical in optimizingsurface transport in composites.

Several advantages are inherent in ion exchange composites over simplefilms. First, composites offer properties not available in simple films.Second, composites are readily formed by spontaneous sorption of the ionexchanger on the substrate. Third, while surfaces dominate manycharacteristics of monolayers and composites, three-dimensionalcomposites are more robust than two-dimensional monolayers. Fourth,interfaces influence a large fraction of the material in the compositebecause of the high ratio of surface area to volume. Fifth, compositesoffer passive means of enhancing flux; external inputs of energy, suchas stirring and applied electric and magnetic fields, are not required.Sixth, local field gradients can be exploited in composites because thefields and molecular species are concentrated in a micro-environmentwhere both the decay length for the field and the microstructuralfeature length are comparable. In some of the composites, the field maybe exploited more effectively than by applying an homogeneous field to acell with an external source.

SPECIFIC EXAMPLES Fuel Cells

Hence it would be very beneficial to achieve high efficiencycompressor/expander power recovery technology. One way to improve theefficiency of the compressor/expander would be to reduce the pressurerequirement. If a passive pressurization process could be providedwithin the fuel cell itself, at no cost to the power output of the fuelcell, power production from present day fuel cells would be increased byapproximately 20%.

Magnetically modified cathodes may reduce the need for pressurization asoxygen is paramagnetic. The field may also alter oxygen kinetics.Potential shifts of +35 mV to +100 mV represent a 5% to 15% improvementin cell efficiency, and comparable savings in weight and volume. Also,in fuel cells, as hydrated protons cross the cell, the cathode floodsand the anode dehydrates. Water transport may be throttled by compositeseparators of graded density and hydration.

Membrane Sensors

Membrane sensors for the paramagnetic gases O₂, NO₂, and NO (the latterrecently identified as a neurotransmitter) could be based on magneticcomposites where enhanced flux would reduce response times and amplifysignals. Sensors for other analytes, where oxygen is an interferant,could distinguish between species by using dual sensors, identicalexcept one sensor incorporates a magnetic field. Examples of thesesensors could be optical, gravimetric, or electrochemical, includingamperometric and voltammetric. In sensors, the measured signal isproportional to the concentration of all species present to which thesensor responds. The presence of a magnetic component in the sensor willenhance sensitivity to paramagnetic species. Through a linearcombination of the signal from two sensors, similar in all respectsexcept one contains a magnetic component, and the sensitivity of themagnetic sensor to paramagnetic species (determined by calibration), itis possible to determine the concentration of the paramagnetic species.In a system where the sensors are only sensitive to one paramagnetic andone diamagnetic species, it is possible to determine the concentrationof both species.

Flux Switches

As nanostructured and microstructured materials and machines developinto a technology centered on dynamics in micro-environments, fluxswitches will be needed. Externally applied magnetic fields can actuateflux switches using electrodes coated with composites made ofparamagnetic polymers and Fe/Fe oxide or other non-permanent magneticmaterial, or internal magnetic fields can actuate flux switches usingelectrodes coated with composites made of electro-active polymers orliquid crystals, where one redox form is diamagnetic and the other isparamagnetic, and organo-Fe or other superparamagnetic or ferro-fluidmaterials or permanent magnetic or aligned surface magnetic fieldmaterial. Also, an external magnet can be used to orient paramagneticpolymers and liquid crystals in a composite containing paramagneticmagnetic beads. Enhanced orientation may be possible with magnetic beadscontaining superparamagnetic of ferrofluid materials.

Batteries

Batteries with increased current densities and power, as well asdecreased charge and discharge times may be made with magnetic beadcomposites. The improvements would be driven by flux enhancement,transport enhancement, electron kinetic effects, or by capitalizing on apotential shift. The required mass of microbeads would little affectspecific power. Since magnetic fields can suppress dendrite formation,secondary battery cycle life may be extended. Examples includemagnetically modified electrodes. The magnetic coatings may be on theelectrodes or elsewhere in the battery structure.

FIG. 17 is a short summary of steps involved in a method of making anelectrode according to two embodiments of the invention. In oneembodiment, the method is a method of making an electrode with a surfacecoated with a magnetic composite with a plurality of boundary regionswith magnetic gradients having paths to the surface of the electrodeaccording to one embodiment of the invention. In particular step 702involves mixing a first component which includes a suspension of atleast approximately 1 percent by weight of inert polymer coated magneticmicrobeads containing between approximately 10 percent and approximately90 percent magnetizable material having diameters at least about 0.5micrometers in a first solvent with a second component comprising atleast approximately 2 percent by weight of an ion exchange polymer in asecond solvent to yield a mixed suspension. Step 708 then involvesapplying the mixed suspension to the surface of the electrode. Theelectrode is arranged in a magnetic field of at least approximately 0.05Tesla, wherein the magnetic field has a component oriented approximatelyalong the normal of the electrode surface and preferably is entirelyoriented approximately along the normal of the electrode surface. Step714 then involves evaporating the first solvent and the second solventto yield the electrode with a surface coated with the magnetic compositehaving a plurality of boundary regions with magnetic gradients havingpaths to the surface of the electrode.

Step 702 can include mixing the first component which includes asuspension of between approximately 2 percent and approximately 10percent by weight of inert polymer coated magnetic microbeads with thesecond component. Alternatively, step 702 can include mixing the firstcomponent which includes inert polymer coated magnetic microbeadscontaining between 50 percent and 90 percent magnetizable material withthe second component. Alternatively, step 702 can include mixing thefirst component which includes inert polymer coated magnetic microbeadscontaining 90 percent magnetizable material with the second component.

In addition, step 702 can include mixing a first component whichincludes a suspension of at least approximately 5 percent by weight ofinert polymer coated magnetic microbeads containing betweenapproximately 10 percent and approximately 90 percent magnetizablematerial having diameters ranging between approximately 0.5 micrometerswith a second component and approximately 12 micrometers. Alternatively,step 702 can include mixing a first component which includes asuspension of at least approximately 5 percent by weight of inertpolymer coated magnetic microbeads containing between approximately 10percent and approximately 90 percent magnetizable material havingdiameters ranging between approximately 0.11 micrometer andapproximately 2 micrometers with a second component.

Mixing step 702 can also involve mixing a first component which includesa suspension of at least approximately 5 percent by weight of inertpolymer coated magnetic microbeads containing between approximately 10percent and approximately 90 percent magnetizable material havingdiameters at least 0.5 micrometers in a first solvent with a secondcomponent comprising at least approximately 5 percent by weight ofNafion in a second solvent to yield the mixed suspension.

Step 702 can involve mixing a first component which includes asuspension of at least approximately 5 percent by weight of inertpolymer coated magnetic microbeads containing between approximately 10percent and approximately 90 percent organo-Fe material having diametersat least 0.5 micrometers in a first solvent with a second componentcomprising at least approximately 5 percent by weight of an ion exchangepolymer in a second solvent to yield the mixed suspension.

Step 708 can include applying approximately between 2 percent andapproximately 75 percent by volume of the mixed suspension to thesurface of the electrode. Alternatively, step 708 can include applyingbetween 25 percent and 60 percent by volume of the mixed suspension tothe surface of the electrode. In yet another approach step 708 caninvolve applying the mixed suspension to the surface of the electrode,the electrode being arranged in a magnetic field between approximately0.05 Tesla and approximately 2 Tesla and preferably the magnetic fieldis approximately 2 Tesla.

An alternative embodiment involving steps 702′ through 714′ (also shownin FIG. 17) involves the use of an external magnetic field. That is,again the method of making an electrode with a surface coated with acomposite with a plurality of boundary regions with magnetic gradientshaving paths to the surface of the electrode when the external magneticfield is turned on. The steps 702 through 714 are then modified intosteps 702′ through 714′ as follows. Step 702′ involves mixing a firstcomponent which includes a suspension of at least 5 percent by weight ofinert polymer coated microbeads containing between 10 percent and 90percent magnetizable non-permanent magnetic material having diameters atleast 0.5 micrometers in a first solvent with a second componentcomprising at least 5 percent of an ion exchange polymer in a secondsolvent to yield a mixed suspension. Step 708′ then involves applyingthe mixed suspension to the surface of the electrode. Step 714′ involvesevaporating the first solvent and the second solvent to yield theelectrode with a surface coated with the composite having a plurality ofboundary regions with magnetic gradients having paths to the surface ofthe electrode when the external magnet is turned on.

FIGS. 18A and 18B show a flux switch 800 to regulate the flow of a redoxspecies according to yet another embodiment of the invention. Inparticular, FIGS. 18A and 18B show an electrode 804 and a coating 808 onthe electrode 804. Coating 808 is formed from a composite which includesmagnetic microbead material 812 with an aligned surface magnetic field;an ion exchange polymer 816; and an electro-active polymer 820 in whicha first redox form is paramagnetic and a second redox form isdiamagnetic, wherein the flux switch is actuated by electrolyzing theelectro-active polymer from the first redox form ordered in the magneticfield established by the coating to the second redox form disordered inthe magnetic field.

Microbead material 812 can include organo-Fe material. The redox speciescan be more readily electrolyzed than the electro-active polymer.Electro-active polymer 820 can be an electro-active liquid crystal withchemical properties susceptible to said magnetic field or anelectro-active liquid crystal with viscosity susceptible to saidmagnetic field. Electro-active polymer 820 includes an electro-activeliquid crystal with phase susceptibility to said magnetic field.Electro-active polymer 812 can include poly(vinyl ferrocenium). Inaddition, the magnetic microbead material 812 can comprise organo-Fematerial.

FIG. 19 shows a dual sensor 900 for distinguishing between a firstspecies (particles A) and a second species (particles B) The dual sensorincludes a first membrane sensor 906 which preferentially passes thesecond species over the first species; and a second membrane sensor 912,which preferentially enhances the concentration of the first speciesover the second species, thereby enabling the measurement of at leastthe first species. The first and second species can be in any state suchas liquid, gaseous, solid and plasma.

In one embodiment, the first species can include a paramagnetic speciesand the second species can include a diamagnetic species. In this case,the first membrane sensor 906 is a magnetically modified membranesensor, and the second membrane sensor 912 is an unmodified membranesensor. The magnetically modified membrane sensor preferentiallyenhances the concentration of and allows the detection of theparamagnetic species over the diamagnetic species and the unmodifiedmembrane sensor enhances the concentration of and allows the detectionof the diamagnetic species and the paramagnetic species, enabling themeasurement of the concentration of at least the paramagnetic species.More particularly, the paramagnetic species can be one of O₂, NO₂, andNO. The diamagnetic species can be CO₂.

In another embodiment, the first species can include a paramagneticspecies and the second species can include a nonmagnetic species. Inthis case, the first membrane sensor 906 is a magnetically modifiedmembrane sensor, and the second membrane sensor includes an unmodifiedmembrane sensor. The magnetically modified membrane sensorpreferentially enhances the concentration of and allows the detection ofthe paramagnetic species over the nonmagnetic species and the unmodifiedmembrane sensor enhances the concentration of and allows the detectionof the nonmagnetic species and the paramagnetic species, therebyenabling the measurement of the concentration of at least theparamagnetic species. More particularly, the paramagnetic species can beone of O₂, NO₂, and NO.

In yet another embodiment, the first species can include a diamagneticspecies and the second species can include a second diamagnetic species.In this case, the first membrane sensor 906 is a magnetically modifiedmembrane sensor, and the second membrane sensor 912 is a differentlymagnetically modified membrane sensor. The magnetically modifiedmembrane sensor preferentially enhances the concentration of and allowsthe detection of the first diamagnetic species over the seconddiamagnetic species and the differently magnetically modified membranesensor enhances the concentration of and allows the detection of thesecond paramagnetic species and the diamagnetic species, enabling themeasurement of the concentration of at least the first diamagneticspecies. The first diamagnetic species can include CO₂.

In yet another embodiment, the first species can be a first paramagneticspecies and the second species can be a second paramagnetic species. Inthis case, the first membrane sensor 906 is a magnetically modifiedmembrane sensor, and the second membrane sensor 912 is a differentlymagnetically modified membrane sensor, wherein the magnetically modifiedmembrane sensor preferentially enhances the concentration of and allowsthe detection of the first paramagnetic species over the secondparamagnetic species and the differently magnetically modified membranesensor enhances the concentration of and allows the detection of thesecond paramagnetic species and the first paramagnetic species, enablingthe measurement of the concentration of at least the first paramagneticspecies. Again, the first paramagnetic species can be one of O₂, NO₂,and NO.

In yet another embodiment of the invention, the first species can be adiamagnetic species and the second species can be a nonmagnetic species.In this case, the first membrane sensor 906 is a magnetically modifiedmembrane sensor, and the second membrane sensor 912 is an unmodifiedmembrane sensor, wherein the magnetically modified membrane sensorpreferentially enhances the concentration of and allows the detection ofthe diamagnetic species over the nonmagnetic species and the unmodifiedmembrane sensor enhances the concentration of and allows the detectionof the nonmagnetic species and the diamagnetic species, enabling themeasurement of the concentration of at least the diamagnetic species.

FIG. 20 shows a cell 201 according to another embodiment of theinvention. In particular, FIG. 20 shows an electrolyte 205 including afirst type of particles. A first electrode 210 and a second electrode215 are arranged in electrolyte 205. The first type of particlestransform into a second type of particles once said first type ofparticles reach said second electrode 215. Second electrode 215 has asurface with a coating 225 fabricated according to the above methods.Coating 225 includes a first material 230 having a first magnetism, asecond material 234 having a second magnetism, thereby creating aplurality of boundaries (33 of FIG. 15A) providing a path between saidelectrolyte 205 and said surface of said second electrode 215. Each ofsaid plurality of boundaries having a magnetic gradient within saidpath, said path having an average width of approximately one nanometerto approximately several micrometers, wherein said first type ofparticles have a first magnetic susceptibility and said second type ofparticles have a second magnetic susceptibility and the first and saidsecond magnetic susceptibilities are different. Coating 225 operates inthe manner described with respect to FIG. 16.

First material 230 in coating 225 can include a paramagnetic species andsecond material 234 can include a diamagnetic species. Alternatively,first material 230 can include a paramagnetic species having a firstmagnetic susceptibility and second material 234 can include aparamagnetic species having a second magnetic susceptibility, and saidfirst magnetic susceptibility is different from said second magnetic. Inyet another approach, first material 230 can include a diamagneticspecies having a first magnetic susceptibility while second material 234includes a diamagnetic species having a second magnetic susceptibility,and said first magnetic susceptibility is different from said magneticsecond susceptibility. In another approach, first material 230 couldalternatively include a paramagnetic species having a first magneticsusceptibility and second material 234 comprises a nonmagnetic species.In another approach, first material 230 can include a diamagneticspecies having a first magnetic susceptibility and second material 234can include a nonmagnetic species. Electrolyte 205 can be anelectrolyzable gas such as O₂ or can include a chloro-alkali.

Numerous and additional modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically claimed.

What is claimed is:
 1. A method for forming a graded density magneticcomposite, comprising: contacting a surface with a compositioncomprising a magnetic material to form a magnetic material coatedsurface; applying to the magnetic material coated surface at least afirst polymer solution comprising at least one polymer, having a firstconcentration, and a first solvent, and evaporating the first solvent toform a first polymer coating on said magnetic material coated surface;applying to the first polymer coating a second polymer solutioncomprising at least one polymer, having a second concentration differentfrom the first polymer solution concentration, and a second solvent, andevaporating the second solvent to form a second polymer coating on thefirst polymer coating, and thereby yield the graded density magneticcomposite.
 2. The method of claim 1, further comprising: applying to thesecond polymer coating a third polymer solution comprising at least onepolymer, having a third concentration different from at least one of thefirst and second polymer solution concentrations, and a third solvent,and evaporating the third solvent to form a third polymer coating on thesecond polymer coating, and thereby yield the graded density magneticcomposite.
 3. The method of claim 2, further comprising: applying to thethird polymer coating a fourth polymer solution comprising at least onepolymer, having a fourth concentration different from at least one ofthe first, second and third polymer solution concentrations, and afourth solvent, and evaporating the fourth solvent to form a fourthpolymer coating on the third polymer coating, and thereby yield thegraded density magnetic composite.
 4. The method of claim 1 wherein theat least one polymer comprises one of a density gradient polymer and anion exchange polymer.
 5. The method of claim 1 wherein the first polymerconcentration ranges from about 2% by weight to about 50% by weight andthe second polymer concentration ranges from about 2% by weight to about50% by weight.
 6. The method of claim 1 wherein applying at least one ofthe first and second polymer solutions comprises spin casting.
 7. Themethod of claim 1 wherein the graded density magnetic composite includesa graded parallel to the surface.
 8. The method of claim 1 furthercomprising removing the graded density magnetic composite from thesurface to yield a free-standing composite.
 9. The method of claim 1further comprising arranging the surface in an external magnetic field.10. The method of claim 9 wherein the magnetic field is non-uniform. 11.The method of claim 9 further comprising removing the magnetic fieldafter applying the at least first and second polymer solutions.
 12. Themethod of claim 9 wherein the at least first and second polymersolutions are applied and the magnetic field is applied such that themagnetic composite includes a magnetic gradient in a first coordinateand a density gradient in a second coordinate.
 13. The method of claim 1wherein the at least first polymer solution comprises a mixture of anion-exchange polymer and a density gradient polymer.
 14. The method ofclaim 1 wherein the at least first polymer solution comprises a densitygradient polymer, and further comprising the step of adsorbing an ionexchange polymer into the density gradient polymer.
 15. The method ofclaim 8, wherein said free-standing composite comprises a film.
 16. Amethod of making a graded density magnetic composite, comprising:forming a magnetic material coated surface; and applying to the magneticmaterial coated surface a plurality of successively formed coatings,each being formed by: applying a respective polymer solution comprisingat least one polymer, having a respective polymer concentration, and arespective solvent, and evaporating the respective solvent, wherein saidrespective polymer concentrations are not all identical, therebyyielding the graded density magnetic composite.
 17. The method of claim16, wherein said respective polymer concentrations are varied to form amonotonic graded density on said magnetic material coated surface withrespect to said at least one polymer.
 18. The method of claim 16,wherein said respective polymer concentrations are varied to form anapproximately discontinuously graded, density on said magnetic materialcoated surface with respect to said at least one polymer.
 19. The methodof claim 16, wherein said at least one polymer comprises at least one ofa density gradient polymer and an ion exchange polymer.
 20. The methodof claim 16, wherein said respective polymer concentration ranges fromabout 2% to about 50% by weight.
 21. The method of claim 16, whereinsaid successively applying said respective polymer solution comprisesspin casting.
 22. The method of claim 16, wherein said graded densitymagnetic composite comprises a graded density parallel to the surface.23. The method of claim 16, further comprising removing said gradeddensity magnetic composite from said magnetic material coated surface toyield a free-standing composite film.
 24. The method of claim 16,further comprising forming said magnetic material coated surface in anexternal magnetic field.
 25. The method of claim 24, wherein saidmagnetic field is non-uniform.
 26. The method of claim 24, furthercomprising removing said magnetic field after said successively applyingsaid respective polymer solution.
 27. The method of claim 24, whereinsaid respective polymer solution is successively applied, and themagnetic field is applied such that the magnetic composite includes amagnetic gradient in a first coordinate, and a density gradient in asecond coordinate.
 28. The method of claim 17, wherein said respectivepolymer solution comprises a mixture of an ion exchange polymer and adensity gradient polymer.
 29. The method of claim 17, wherein saidrespective polymer solution comprises a density gradient polymer, andfurther comprising the step of adsorbing an ion exchange polymer intothe layer formed by the density gradient polymer.